The present invention relates to the determination of Universal Coordinated Time from Loran-C transmissions.
The transmission times of Loran-C pulses are precisely controlled relative to the ensemble of atomic frequency standards maintained by the United States Naval Observatory (USNO) in Washington, D.C. This network of USNO clocks is in turn compared with those of other international timekeeping laboratories such as the Bureau International des Poids et Mesures (BIPM) in Paris, France and the National institute of Standards and Technology (NIST) in Boulder, Colo. Using this global approach to clock intercomparison, along with sophisticated clock weighting algorithms, these and other laboratories contribute to the existence and maintenance of Universal Coordinated Time (UTC).
Due to the pulse modulation scheme of Loran-C, skywave phase skew is avoided if excessive delay in front-end band pass filters is avoided and only the first thirty microseconds of the pulses are used. Modern Loran-C receivers are able to identify the zero crossing at the end of the third cycle of a Loran-C pulse, and track it with 100 nanosecond RMS error assuring optimum time of arrival measurement stability.
Based on the precise transmission times of the Loran-C pulses along with the ability of Loran-C receivers to accurately track the correct cycle, transfer of UTC to remote locations is made possible with knowledge of two parameters: 1) the next transmission Time of Coincidence (TOC) of a Loran-C pulse group with a UTC second (a periodic event occurring every Chain Coincidence Interval (CCI) seconds); and 2) the combined knowledge of UTC, receiver delay and propagation path delay accurate to less than one Group Repetition Interval (GRI) of the Loran-C pulses.
Given an accurate position and careful propagation path modeling, the attainable time transfer accuracy via Loran-C is limited to the one to two microsecond level of inaccuracy in the path model. Depending on the range to the transmitter and the type of terrain between the receiver and transmitter, the stability of the Loran-C arrival times may vary from 100 to 500 nanoseconds, RMS.
All Loran-C chain timing is referenced to 00:00:00 hours (UTC), Jan. 1, 1958, the common epoch for all Loran-C transmissions. Each Loran-C chain broadcasts at a particular GRI. As specified, these GRI's may vary from 0.04 to 0.09999 seconds in increments of 10 microseconds. However, no Loran-C chains have made use of the 10 microseconds resolution in GRI, and all have been restricted to GRI's which are integer multiples of 100 microseconds.
The fundamental period of the transmissions is doubled to twice the GRI by application of the Phase Code to the eight (nine for master transmitters) pulses within each group. An alternating Phase Code either A or B is applied to each pulse group. Phase Code A began at the epoch, in the first GRI of each Loran-C chain, and is applied in all odd GRI's. Phase Code B began one GRI later in the second GRI of each Loran-C chain and is applied in all even GRI's.
By knowing this common epoch, the GRI, and the number of UTC leap seconds which have occurred since the epoch, the TOC's of the Loran-C transmissions with the UTC seconds may be computed. At the same time, it can also be determined whether the TOC will be with an even or odd Phase Coded GRI. Since a Loran-C receiver can determine from the Phase Code whether an even or odd GRI has been received, if the combination of UTC time and propagation path delay is known to within one GRI, then the Loran-C pulse group coincident with the UTC second may be unambiguously detected. The detection of this coincident transmission then allows, as described above, transfer of UTC to the one microsecond level of accuracy. In this way, knowing correct real time to a relatively course accuracy, (40 to 99.99 milliseconds), the Loran-C TOC synchronization technique refines this initial time accuracy to the one microsecond level (assuming an accurate position and careful receiver delay calibration and propagation path modeling).
By using a single Loran-C transmission, knowledge of accurate position is not crucial to the determination of the proper pulse group since the maximum propagation path delay for usable transmitters is typically less than 10% of the GRI. Unambiguous coincident pulse group determination is dominantly dependent on the initial UTC accuracy. Of course, the ultimate accuracy of the transfer is directly dependent upon how accurately the receiver position is known.
However, this single Loran-C transmission TOC synchronization technique requires the provision of real time to the receiver with less than 100 milliseconds of error from UTC. Such accuracy requires some external real time clock having a high level of sophistication. Providing such accurate time to the receiver is a recurring problem any time loss of power to the receiver has occurred.
It would therefore be desirable to relax the accuracy requirements of the real time clock maintained at a receiver location, while preserving the ability to synchronize time maintained at the receiver to UTC.